1. Field of the Invention
The present invention relates to a multi-beam klystron apparatus which amplifies radio-frequency power.
2. Description of the Related Art
There is known a klystron apparatus comprising a klystron main body which includes an electron gun unit which generates an electron beam, an input unit which inputs radio-frequency power, a radio-frequency interaction unit which amplifies the radio-frequency power by an interaction between the electron beam and a radio-frequency electric field, an output unit which outputs the radio-frequency power from the radio-frequency interaction unit, and a collector unit which collects the electron beam which has been used; and a converging magnetic field device which is mounted on the klystron main body and converges the electron beam.
The converging magnetic field device, in general, adopts such a structure that several to several tens of electromagnets, which generate lines of magnetic force in the axial direction of the klystron, are arranged main body. Annular coils of the electromagnets are disposed around the klystron main body. The electron beam is converged by electric fields which are generated by an electric current that is let to flow through the coils. The directions of the magnetic fields are the same on the axis of the electron beam from the cathode of the electron gun unit to the collector unit. The magnetic flux density is substantially uniform in the radio-frequency interaction unit. The magnetic flux density in the vicinity of the output unit is increased to some degree so as to prevent divergence of the electron beam. The magnetic flux density in the electron gun unit and collector unit is much lower than that in the output unit, and is set at a proper value.
An electron gun unit of a klystron apparatus, which uses a single electron beam, generally adopts magnetic field immersion type convergence. In this case, the diameter of an electron beam passage hole in a pole piece which is a magnetic pole on the input side of the radio-frequency interaction unit, the distance to the cathode of the electron gun unit, and the shape of the wehnelt of the electron gun unit are properly set. Thereby, the electron beam is converged in an inverse fashion, relative to the divergence of the line of magnetic force leaking from the hole in the pole piece.
It is generally known in the present technical field that the output conversion efficiency of the klystron apparatus becomes higher as the ratio of a beam current to a beam voltage, which is called “perveance”, becomes smaller. It is also known that one means for achieving higher efficiency is a multi-beam klystron apparatus wherein the number of electron beams is increased from one in the prior art to several to several tens, the perveance of each electron beam is set at a low value, the beam voltage that is applied to the electron gun unit is set at a low value, and a totally high output conversion efficiency is obtained.
In a converging magnetic field device of this multi-beam klystron apparatus, a plurality of holes for passing a plurality of electron beams are formed at locations apart from the center axis of the pole piece. Thus, lines of magnetic force, which leak to the electron gun unit from the holes formed at locations apart from the center axis of the pole piece, diverge as the distance increases from the converging magnetic field device. Hence, it is necessary to arrange the cathodes of the electron gun unit in a three-dimensional fashion, for example, in a spherical fashion. In addition, in a magnetic field distribution that is used for ordinary magnetic field immersion type convergence, it is necessary to provide the pole piece with very large holes, relative to the beam diameter of the electron beam. As a result, an electron beam passing through a hole in the pole piece is affected by magnetic fields leaking from holes for other electron beams.
For example, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-16504 (pp. 3-5, FIGS. 1-3), there is a multi-beam klystron apparatus wherein cathodes of an electron gun unit are arranged in a plane, and not in a spherical fashion, thus giving consideration so as to generate a magnetic field that is suited to convergence of each electron beam. In this case, a shared pole piece, which is a magnetic body of, e.g. iron-nickel-cobalt type, is disposed near the cathode of the electron gun unit. In addition, a single magnetic field generating element, which projects to the electron gun unit side, is disposed on the converging magnetic field device. Further, at least one stage of an additional pole piece and a magnetic field generating element is provided.
In the above-described multi-beam klystron apparatus wherein consideration is given so as to generate a magnetic field that is suited to convergence of plural electron beams, a radial-directional component occurs in the magnetic field that is generated by the single magnetic field generating element projecting from the converging magnetic field device toward the electron gun unit. Consequently, the electron beam is deflected.
Besides, the structure becomes complex. In particular, since the shared pole piece is disposed in the electron gun unit, the electron gun unit becomes complex. The cathode of the electron gun unit is operated normally at about 100° C. As a result, the temperature of the wehnelt of the electron gun unit, as well as the cathode, rises to several-hundred ° C. The shared pole piece is disposed at a location that is immediately near the cathode and is surrounded by the wehnelt. With an increase in heat capacity of the vicinity of the cathode, there may occur an increase in heater power, an increase in discharge gas, and thermal deformation due to different kinds of metals. In consideration of the thermal deformation of structural components of the electron gun unit, it becomes difficult to design and manufacture the structural components in the normal-temperature state.
The on-axis magnetic field distribution on the beam axis is so adjusted as to become substantially equal to that of an ordinary single-beam tube. Hence, it is not possible to ignore the adverse effect on an electron beam passing through the hole in the pole piece due to the magnetic field leaking from the holes for other electron beams. A counter-measure to this is required.
The present invention has been made in consideration of the above-described problems, and the object of the invention is to provide a multi-beam klystron apparatus wherein lines of magnetic force, which are parallel to a center axis, are obtained near an electron beam axis apart from the center axis, without making an electron gun unit complex.